The FX2 8051 USB Firmware is pretty basic. The firmware (montly written in C) main job is to configure the FX2. After the code is downloaded it will setup the endpoints and any other configuration that is needed. The FX2 is a pretty sophisticated device. If you take a gander at the user manual (trm, techinical reference manual, available at Cypress website) you will see that it has a lot of operating modes and registers that can be configured.
Other than configuring the endpoints the firmware will setup the interrupt vectors
The FX2 has 6 endpoints, 0, 1, 2, 4, 6, and 8. Endpoint 0 is always the control endpoint. Endpoint 1 can be used by the 8051 controller. That is the intended mode. Most these projects the FX2 is used as a conduit to transfer data from USB to an FPGA. Not a lot of data is sent to the 8051. In some cases control type data (not in need of high bandwidth) will be sent to the 8051. The 8051 will only use endpoint 0 (required) and endpoint 1. The remaining endpoints are connected to the slave fifos.
Here is a list of the vendor requests implemented (or TBI).